# Function: tap()

> 

> **tap**\<`TValue`, `TError`\>(`observer`): `MonoTypeOperatorFunction`\<`TValue`, `TError`\>

Defined in: [packages/server/src/observable/operators.ts:140](https://github.com/trpc/trpc/blob/7c907acfd8b31929214792e3b8c2ad7a8ac72563/packages/server/src/observable/operators.ts#L140)

Calls `observer` with each value, error and completion, and passes them on unchanged.

## Type Parameters

| Type Parameter |
| ------ |
| `TValue` |
| `TError` |

## Parameters

| Parameter | Type |
| ------ | ------ |
| `observer` | `Partial`\<[`Observer`](https://trpc.io/docs/typedoc/server/observable/interfaces/Observer.md)\<`TValue`, `TError`\>\> |

## Returns

`MonoTypeOperatorFunction`\<`TValue`, `TError`\>